On Fri, 11 Feb 2000 11:44:35 -0600, "Bob Hughes"
<omn### [at] hotmailcom?subject=PoV-News:> wrote:

>I like it, mainly because of the control over scene illumination.
>You're suggesting a frosted filter effect having both a threshold and a
>tolerance then I suppose.  

That's one way to think of it. I was actually thinking of setting a
minimum and maximum value, instead of the single  one-parameter
"threshold", as first suggested. Perhaps "post_process_min" and
"post_process_max", or some similar syntax.

To specify  a threshold, and then specify a (+-) tolerance amount for
that threshold would achieve about the same thing. It seems to be just
a matter of which syntax one might prefer to achieve the same result.

>Better yet would be a user-defined filter way of
>doing it, perhaps an array describing the baseline method to use.  Something
>similar to what I've seen in paint programs which might allow for a strength and
>pattern to be set.  If anyone knows what I'm talking about.

I think I know what you mean. I have used paint programs that allow
users to set a grid of multiplier values to create a custom filter. 

Something like this creates a strong blur filter::

   1    1    1    1    1
   1    1    1    1    1
   1    1    1    1    1
   1    1    1    1    1
   1    1    1    1    1

   Divisor: 25
   Offset: 0

This, would create a sharpen filter:

   0     0    0    0    0
   0   -2   -2   -2    0
   0   -2  32   -2    0
   0   -2   -2   -2    0
   0     0    0    0    0

   Divisor: 16
   Offset: 0

Blur, sharpen, linear blur(streaking), bas relief, find edges,
negative effects, contour, edge enhancement, and other effects can be
achieved in this manner. I've used a 5x5 matrix for this example, but
larger matrices can sometimes achieve more dramatic or accurate
effects. They also require more computations. Sometimes it is also
desireable to perform these matrix operations upon only one color
channel, or to supply a different matrix for each channel. This might
especially be handy for people working with an alpha channel.

I also wanted to mention that the contour and edge effects can be
played with to achieve some cartoon or line drawing effects. This
might come in handy for the post-process cartoon effect Nathan was
working on.

I assume that Nathan's vision of Mega-POV will allow several
post-process effects to be appled in sucession, if desired. It would
be nice to set the post-process "thresholds" or "ranges" seperately
for each post-process effect performed, instead of having some global
variables that controlled all the post-processing within the scene
file. For example, one might want to focal blur the whole scene, but
also have added blur only on the strongest of highlights.

later,
Glen Berry

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